The need for long term ocean floor observatories for the purpose of solid earth studies has now been widely recognized and several national and international efforts are underway to resolve the challenging technological issues associated with such deployments. At the regional scale, California is in a particularly good position to benefit from the installation of off-shore permanent broadband seismic stations, to complement the land based network. Indeed, most of the existing stations are located on the east-side of the north-America/Pacific plate boundary and offer limited azimuthal coverage for moderate to large regional earthquakes on the main active faults. They also provide limited location and source mechanism resolution for infrequent and generally smaller (M<4) off-shore earthquakes on faults, such as the San Gregorio, that are known to have experienced significant (M6) events. Finally, off-shore broadband recordings of on-land earthquakes would provide much needed data to constrain the three-dimensional structure of the crust and thereby improve our understanding of the nature and evolution of this plate-boundary. Since large regional events are rare, deployment of ocean bottom stations for long periods of time (years) is necessary for this purpose. There are several technical challenges that need to be addressed before permanent ocean bottom seismic observatories are deployed routinely: issues of optimum installation (borehole versus ocean floor), means of installation (manned or unmanned vehicles), sources of power supply and means of data retrieval are among the most critical ones.
One of the key issues in the design of these ocean bottom observatories is how to optimize installation in order to minimize the background noise in the frequency bands most relevant to the recording of regional and distant earthquakes.
In the summer of 1997, we conducted a pilot, target of opportunity international experiment in Monterey Bay (MOISE, Monterey Bay Ocean Bottom International Seismic Experiment (Stakes et al., 1997; 1998), associating the resources and expertise of three teams: MBARI (Monterey Bay Aquarium Research Institute), a multi-institutional team from France (IPG Paris; DT/INSU: Division Technique de l'Institut National des Sciences de l'Univers; Université de Bretagne Occidentale, Brest), and the Seismological Laboratory at UC Berkeley. The primary goal of this experiment was to demonstrate the feasibility of deployment and intermediate term operation of several geophysical packages on the seafloor, using a remotely operated vehicle (ROV). The installation was successful, and 3 months of broadband seismic and various auxiliary data were acquired and processed (06/18/97-09/12/97).
The site of the MBARI experiment was chosen in February 1997, on a flat plateau area called Smooth Ridge in Monterey Bay, just beyond the active San Gregorio fault 16.1, 40 km off-shore, at a water depth of 1015m. The deployment of the scientific packages was performed using the MBARI ship "Point Lobos" and the MBARI remotely operated vehicle (ROV) "Ventana", with maximum operating depth of 1850 m. The location of the experiment was accessible within 2 hours transit time from Moss Landing (CA), the MBARI headquarters.
The central instrument package of MOISE was a 3 component broadband seismic package composed of Guralp CMG-3 sensors mounted on leveling gimbals. The electronics were adapted for this ocean floor deployment by DT/INSU. The seismometers had been previously deployed in the mid-atlantic ocean, during the OFM experiment (Observatoire Fond de Mer; Montagner et al., 1994). The seismic package includes a 16-bit, gain-ranged digitizing system, an automatic re-centering system as well as a self-leveling system driven by a small cpu. The cylindrical aluminum housing was designed by the DT/INSU and outfitted with handles for manipulation by the ROV and the male side of an 8-pin Nautile connector. During the deployment, the seismic package was connected underwater to a lithium battery package as well as to the recording system. The recording system was designed and built by Scripps Institution of Oceanography (SIO) for ocean bottom applications (Low-cost Hardware for Earth Applications and Physical Oceanography, "L-Cheapo", J. Orcutt, personal communication). The L-Cheapo software was modified for the MOISE experiment so that it would allow simultaneous connections to the seismic sensor package and the ship. In this way, it was possible to establish communication with the sensor package directly from the ship, which was done three times: at installation and retrieval time, and, also, one month after installation, to verify the good functioning of the seismometers and data recording. During this intermediate visit to the site, the ROV established a connection from the ship to the L-Cheapo recorder, and it was possible to retrieve samples of data, as well as connect a lap-top computer aboard the ship to the seismometer system, in order to relevel and recenter the seismometers. The software also allowed us to instruct the seismometer, at the beginning of the experiment, to unlock the gimbals once installed, and, at the end of the experiment, to shut down and lock the gimbals.
In order to achieve good coupling of the seismometer system with the ground, efforts were made to partially bury it in the sediments. Site preparation included placing a large weight to compact the sediment several weeks before deployment. At the time of deployment, a PVC cylinder was first brought to the site and 75% buried in the soft sediment, which was removed from inside the cylinder using a shovel manipulated by the ROV. We had trouble digging the hole much deeper than half a meter because of the cohesive nature of the organic and clay rich sediment. The seismometer package was then lowered into this hole by elevator. The gap left between the seismic package and the PVC cylinder was filled with 3mm glass beads to consolidate the installation and allow easy recovery at the end of the experiment. The seismic package had a rather elongated shape, partly because the underwater connectors were mounted on its top and, once installed, it was protruding about 70cm above the sea-floor, thus presenting an unfortunately large surface to the bottom ocean currents.
In addition, a CTD/pressure gauge (on loan from Curt Collins at the Naval Postgraduate School), with a support frame and anchor, as well as an S4 current meter, were deployed on a short mooring in the vicinity of the seismic package for the entire duration of the experiment. These were stand-alone instruments. In particular, the current meter data proved invaluable in understanding the sources of seismic background noise during this deployment. A self-contained electromagnetic package containing magnetometers, electrometers and an internal data logger designed at UBO (Montagner et al., 1997) was deployed at the same site during the same time period. The resulting data will be described in a separate publication. Finally, several other instruments were deployed in Monterey bay during the same time, as part of a separate experiment (MBARI Margin seismology project): 3 standard three channel 4.5 Hz OBS's and a single channel hydrophone with an independent L-Cheapo recorder, to monitor microseismic activity of the San Gregorio and Monterey Bay Fault zones.
Three component broadband seismic data were acquired continuously on the ocean floor from 06/21/97 through 09/11/97, at a sampling rate of 20 samples/sec. Several regional earthquakes of magnitude 3.5 and larger as well as several large teleseisms were well recorded during that time period. Comparison of the records obtained at the ocean-bottom site (station name: MOIS) with those of near-by land sites of the Berkeley Digital Seismic Network (BDSN) and of the Geoscope Network (SCZ) provide useful insight into the quality of the data. 16.2 shows an example of recording at MOIS of a teleseismic event, compared to those of BDSN station FARB, a noisy island site, as well as SAO, the closest BDSN continental site. The regional event data are raw, but the teleseismic ones were band-pass filtered between 5 and 50 sec, a frequency band of minimum noise (or low-noise "notch") as documented from previous ocean-bottom experiments (e.g. Webb et al., 1991; Montagner et al., 1994). Background noise levels were found to fluctuate considerably in this period band and, not surprisingly, the best recordings were obtained during the quietest periods, when noise in the minimum noise window was comparable to that observed commonly at nearby land sites.
We owe the success of the MOISE experiment to the outstanding efforts of the technical team, which also included Jean-Claude Koenig, Jean Savary at DT/INSU, Paul McGill and Craig Dawe at MBARI. Karen Salamy at MBARI assisted with video and various preparation and data processing aspects of the experiment. B. Romanowicz thanks L. Breger, K. Galdamez, L. Gee, C. Megnin at UC Berkeley and P. McGill at MBARI for help with graphics.
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